In ink jet recording, small ink droplets are ejected by one of a variety of different mechanisms so that they adhere to a recording paper on which they form dots. Apart from the fact that this method is less noisy than dot impact recording, it is easy to achieve full color images and high speed printing is possible. However, the inks used in ink jet recording are normally water-based inks that employ direct dyes or acidic dyes, and therefore have poor drying properties.
The properties required of an ink jet recording paper used in this ink jet recording method are:
(1) high ink drying speed, PA1 (2) high optical density of image, PA1 (3) the ink does not overflow or blur, PA1 (4) the paper does not crease due to absorbing ink, PA1 (5) the print is highly waterproof.
Now that the color technology of ink jet printers is improving and their prices are decreasing, the use of color ink jet printers by individual consumers is rapidly becoming more widespread.
These color printers reproduce a variety of colors by combining inks of the single colors cyan, magenta, yellow and black inks, consequently the amount of ink adhering in the mixed ink areas is from 2 to 3 times as much as the amount of ink adhering in the case of monochrome printers. Hence, when ordinary non-coated papers, such as the kind of paper used in conventional monochrome printers, are used in a color printer, the amount of ink absorbed is inadequate resulting in strike through or overflow. Conventional heavily coated type papers, on the other hand, were difficult to handle and did not have the texture of ordinary paper. There was therefore a demand for a lightly coated paper. To improve the waterproof properties of print produced by ink jet printers, the water-soluble dyes used in inks are made more difficultly soluble by replacing sulfo groups in the dye with carboxyl groups (R. W. Kenyon, 9th International Congress on Advances in Non-Impact Printing Technologies/Japan Hardcopy '93, p. 279 (1993)).
As carboxyl groups are usually weakly acidic, under alkaline conditions dissociation is promoted so that the dye dissolves, but under relatively strongly acidic conditions, it is present as a free carboxylic acid so that dissolution is prevented. The improved waterproof properties of the dye are due to this principle. The dye is dissolved in ink of comparatively high pH, but after printing when the dye adheres to paper, as the pH of the paper surface is relatively low, the dye is present as the free acid and is therefore rendered difficultly soluble. Such dyes which have been rendered difficultly soluble are described together with their chemical structure in the aforementioned reference in the literature, and they all possess carboxyl groups.
Of these dyes, some possess both carboxyl groups and sulfo groups, but it is the solubility of the carboxyl groups which varies due to the change of pH before and after printing. Since dyes which possess carboxyl groups react strongly with alkaline earth metal ions, changes of color rendering properties easily occur, and salts which are difficultly soluble in water are easily formed.
In the event of such a change of color rendering properties, the print quality of printed documents obviously deteriorates, and if a difficultly soluble salt is produced, a metallic gloss appears which also impairs print quality.
In recent years, the use of neutral paper has become more widespread replacing the acidic paper which was mainly used conventionally. This neutral paper comprises calcium carbonate as a filler, and is known as calcium carbonate paper. When the aforesaid water-resistant inks were used on this neutral paper, it was therefore a frequent occurrence that the calcium carbonate in the paper reacted with the aforesaid dye comprising carboxyl groups, causing a change of color rendering properties and a deterioration of print quality.
When the inventors attempted to improve these defects by providing a recording layer on neutral paper, they found that even in the case of a coated paper using calcium carbonate paper as a base paper, a lightweight coating of approximately 7 g/m.sup.2 or less did not suffice to completely cover the base paper so that the same deterioration of print quality occurred as mentioned hereinabove.
They found moreover that when a relatively strongly alkaline salt such as calcium carbonate was used as a filler, the carboxyl groups in the dye tended to dissociate even if the calcium carbonate did not react with the pigment so that the improvement of waterproof properties was not as great as had initially been expected. There was also a disadvantage in that the dye penetrated the paper so that optical density decreased.
The inventors discovered as a result of intensive studies that when recording was performed using water-soluble inks comprising mainly carboxyl groups as hydrophilic functional groups, an ink jet recording paper which provided high print quality and printed materials having excellent waterproof properties could be obtained even with a lightweight coating by avoiding the use of calcium carbonate as filler and using a base paper comprising kaolin and/or illite.